The present invention relates generally to systems for dispensing cryogenic fluids and, more particularly, to a self-contained system for dispensing liquid natural gas and compressed natural gas.
Economic and environmental concerns have resulted in widespread efforts to develop fuel substitutes for gasoline and diesel fuel. Natural gas, whose main component is methane, presents a viable alternative to gasoline and diesel fuel because it is relatively inexpensive, burns cleanly and produces emissions which are much less harmful to the environment. Both compressed natural gas (CNG) and liquid natural gas (LNG) have found use as alternative fuels in vehicles. Accordingly, it is desirable to have a system that can dispense both CNG and LNG.
LNG typically must be conditioned prior to dispensing so that it is in a saturated state at the pressure required by the vehicle to which it is being dispensed. In addition, LNG is typically dispensed from a dispensing station storage tank to a vehicle tank by pressurized transfer. It is desirable for this transfer to take place as quickly as possible so that a patron of the dispensing station does not have to wait for an extended period of time during refilling.
Historically, gases and liquid have been transferred rapidly between containers by making a big pressure differential between the fluid storage tank and the tank that is being filled (the receiving tank). There are typically two ways of doing this. The first is by starting out with the storage tank at a higher pressure than the receiving tank and then allowing this pressure to force the gas or liquid into the receiving tank. In so doing, product is transferred, but the pressure in the storage tank drops to the point where the pressures of the two tanks become equal and nothing more is transferred. Transfer can continue by using additional storage tanks, until they too equilibrate with the receiving tank. Such “cascade filling” is well known in the CNG industry. After use, the CNG storage tanks are typically slowly refilled with a compressor. While cascade filling works well in dispensing CNG, filling multiple tanks with liquid and then conditioning and pressurizing them is inefficient. As a result, cascade filling is not optimal for the rapid dispensing of LNG.
The second way of creating a large pressure differential between tanks so that fluid is rapidly transferred is to push liquid out of the storage tank by rapidly applying pressure to, or building pressure in, the head space of the storage tank. The gas required to create this pressure can come from an outside stored source, as in U.S. Pat. No. 6,044,647 to Drube et al., or can by found by vaporizing part of the liquid in the storage tank and turning into a vapor, as in U.S. Pat. No. 5,231,838 to Cieslukowski.
While the systems of the Drube et al. '647 patent and Cieslukowski '838 patent function well in dispensing LNG, they are unable to simultaneously dispense CNG. In addition, both systems, as with many prior art systems, require more than one heat exchanger to operate. This adds to system complexity and cost.
U.S. Pat. Nos. 5,421,160 and 5,537,824, both to Gustafson et al., disclose systems that can dispense both LNG and CNG. The systems of both of these patents, however, use compressors to compress the natural gas prior to storing it. This is a disadvantage as compressors introduce additional complexity, expense and maintenance requirements. In addition, each system also requires two heat exchangers which, as described above, also adds to system complexity and cost.
Pilot programs for testing and demonstrating the viability of LNG or CNG as fuel alternatives require pilot dispensing stations which are capable of efficiently storing large amounts of LNG and/or CNG and dispensing it to a fleet of vehicles. Because of the different storage requirements for LNG and conventional fuels, it is impractical and economically unfeasible to modify existing gasoline distribution facilities for LNG. It is therefore desirable to minimize the capital investment in site improvements required to install LNG and/or CNG pilot dispensing stations since it is difficult to recapture such outlays during the relatively short life of the facility. It is therefore also desirable to provide an LNG and CNG dispensing station that is portable and self-contained to permit quick transport and installation at distribution sites.
In prior art LNG dispensing systems, the storage tanks from which the LNG transfer to vehicles is made are traditionally filled by gravity. By opening a valve on the top and the bottom of the storage tank, liquid pours into it from a bulk tank or some other source. The valves are then closed, the liquid is conditioned to the right saturation point by bubbling a warm gas though it, and then an artificial pressure is created on the liquid with gas pressure to force it out of the tank.
An issue exists, however, as to how to create a method to fill the storage tank in a confined, height limited space. Such a situation may occur, for example, with a self-contained station positioned inside a 40 foot ISO container. Such an environment does not provide enough height to gravity fill the storage tank.
Accordingly, it is an object of the present invention to provide a system that can efficiently condition and rapidly dispense liquid natural gas.
It is another object of the present invention to provide a system that can dispense both liquid natural gas and compressed natural gas.
It is another object of the present invention to provide a system that can produce and dispense compressed natural gas without the use of a compressor.
It is still another object of the present invention to provide a system for dispensing compressed natural gas and liquid natural gas that is economical to construct and maintain.
It is still another object of the present invention to provide a system for dispensing compressed natural gas and liquid natural gas that will fit in a compact and portable space.